Echinoderms are well known to just about everyone who has been down to the sea thanks to their great publicists - the sea-stars or starfish. They are a unique group as they have 5-rayed symmetry or pentametry which extends to all members of the group though may be multiplied to 10, or 15 or more. All members are marine dwelling.

Echinodermata - Echinoderms Echino - spiny, Derm - skin

Echinoderms are one of the oldest animal groups, dating back over 500M years, and as a consequence have a number of characteristics that are unusual amongst animals. They were abundant in the distant past and fossilize well due to their calcium carbonate skeleton, there are around 7,000 living species and 13,000 described extinct species.


Typical Echinoderms

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Red knobbed Starfish, Protoreaster linckii
A five armed starfish represents the group more than any other member, in the human mind at least. Despite being colourful and knobbly it is perfectly clear what it is.

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A group of small yellow sea cucumbers
picture Nick Hobgood used under CC3 Attribution ShareAlike Unported License

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Two species of sea urchin, West Indian Sea Egg, top Tripneustes ventricosus and Reef Urchin - bottom, Echinometra viridis
picture Nick Hobgood used under CC3 Attribution ShareAlike Unported License


Cool Echinoderms

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A sea star, Asterias rubens eating a mussel
Note how the mussel has been almost surrounded and the tube feet  that are used to pull open a small gap between the two shells so the sea star can evert its stomach to start digesting the contents.
picture Betacommandbot, used under CC3 Attribution ShareAlike Unported License

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A Crinoid or Feather Star
Often attached to a substrate as juveniles with a (very elgant) free swimming ability as adults.

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Pedicellaria, many echinoderms have these structures covering their body, the central image is a section through one showing three tooth-like fangs with poison glands below, the one to the left is more forcep-like. The largest of these structures are about 1mm across. It is thought that their main function is to nip at algae and animal larvae of encrusting organisms that might try to settle on the outside of the echinoderm. In some starfish however they can aid in food capture by grasping small fish and shrimps that rest on the echinoderm.

Basic Features:

  • Pentametry - 5 rayed symmetry - the distinguishing feature of the Echinoderms
  • A water vascular system, a network of water filled channels that often end in suckered tube feet (podia in large numbers) that enable them to move about and bring objects to the mouth
  • Endoskeleton made of many calcareous (bony) pieces or ossicles
  • No brain, heart or eyes, though absence of structures is not a useful characteristic for identification
  • Only found in the sea, worldwide and at all depths
  • Reproduction can be sexual or asexual

Kingdom - Animalia
Phylum - Echinodermata

The group includes:

  • Class Asteroidea - Sea Stars / Starfish. Free moving echinoderms where the body is comprised of rays or arms radiating from a central disc. usually 5 armed, but can be 7-14 in some species and up to 40 in others. Podia used for movement.
    Sea Star or Starfish? Marine scientists are embarked on the long slow process of changing the traditional name starfish to sea star because they aren't fish.
  • Class Ophiuroidea - Brittle Stars, also called basket stars or serpent stars. Arms are relatively long and thin, podia not much used for movement. Typically 5 arms though these may branch away from the central disc.
  • Class Echinoidea - Sea Urchins and sand dollars. Circular or oval in shape without arms. Body covered in spines and the ossicles are fused into a solid case, the test.
  • Class Crinoidea - Feather Stars.
  • Class Holothuroidea - Sea Cucumbers. Elongated and lie on their side, no arms. Skeleton reduced to microscopic ossicles, so soft-bodied  but have a tough outer skin.

 

What do echinoderms eat?

Starfish have their mouth on the underside of the center of the disc, the digestive system consists of a large stomach that occupies much of the central disc and then extensions that project along each of the arms.

Starfish are carnivores and scavengers. They feed on all sorts of other invertebrates, particularly bivalves, snails, crustaceans, marine worms, other echinoderms and even fish. They are attracted to the bodies of dead animals on the sea floor. Some are very specific in their eating habits and will only eat sea cucumbers for instance, others are more adaptable and will eat a wide variety of prey.

Starfish with short inflexible arms ingest their prey whole before digesting them. Those with longer and more flexible arms are able to turn the stomach inside-out through the mouth, this is how starfish manage to feed on bivalves such as mussels and oysters. The starfish prises open the shells using its arms and tube feet and then everts part of its stomach through the gap, only a narrow gap is needed with some species only needing a gap of 0.1mm. The hold can then be relaxed and the shell allowed to close trapping the stomach which comes to no harm. Secreted enzymes start to digest the muscle holding the two halves of the shell together and the mollusc is consumed. This process takes from two and a half to eight hours depending on size.

Suspension, deposit and filter feeding. These are common feeding methods of the Echinoderms with some starfish and most brittle stars, feather stars and sea cucumbers all obtaining food by one or more of these methods. The tiny tube feet are involved in passing collected food towards the central mouth. These animals are detritivores performing similar functions to earthworms on the land in recycling nutrients.

Sea Urchins have a highly developed feeding structure called Aristotle's lantern. This is a five sided pyramidal shaped structure with teeth at the point that faces downwards from the middle of the sea urchins body. It can be extended to some degree and swung from side to side to some extent too. Sea urchins are mostly grazers of algae that covers rocks or of larger marine plants such as grasses and kelp though some will feed on corals while others feed on minute food particles in sand.

 

What eats echinoderms?

Sea otters are keen on sea urchins, as are fish, starfish and people. Sea urchin eggs are eaten in many parts of the world, especially Japan. Sea urchins however are generally difficult to eat as many species have long pointed and brittle spines that will break off easily in a wound. Some particular types of fish though can cope with them and are probably their biggest predators.

Starfish are eaten by fish such as some rays and sharks, they are also eaten by other larger starfish and some predatory molluscs such as large snails.

Sea cucumbers are farmed in parts of SE Asia, particularly China where they are considered to be delicacies or are used in traditional medicines (cucumber refers to their shape not their taste). Other than this, fish and turtles are the main predator, though they have a defence mechanism which involves ejecting part of their digestive systems when attacked as a sacrifice for the greater good. The predator then eats these parts and leaves the rest of the sea cucumber alone to re-grow the missing parts.

Brittle and feather stars, no-one is really sure though as they live in the sea it's a good guess that fish eat them.



Sea urchins, sea stars, brittle stars and sand dollars - BBC, Planet Earth



Swimming feather star


Sea cucumbers at 2324m below the surface


Top banner - Sea urchin, Heterocentrotus trigonarius, picture courtesy David Burdick NOAA.